An isolated MOSFET switching circuit, as seen in FIG. 1, uses an opto-battery 100, as described in more detail below, to provide isolation and power to the gates of the drive metal-oxide-semiconductor field-effect transistors (MOSFET) 102 and 104, forcing the MOSFETs 102 and 104 to turn on. MOSFETs 102 and 104 are high voltage MOSFETs. These MOSFETs have a gate-source capacitance, shown as capacitors 106 and 108 for clarity in FIG. 1, and also referred to as capacitors Ca and Cb, respectively. To turn the isolated MOSFET switching circuit in FIG. 1 on, switch 110 closes to provide 10-15 mA of current through the light emitting diode (LED) 112 of the opto-battery 100. The light from the LED causes a small current (e.g., 10-30 μA) to flow in the current source of the opto-battery 100. The current flows into capacitors 106 and 108, increasing the voltage across them at a rate of dV/dt=i/(Ca+Cb). This rate continues until the zener-diode 114 zeners, which is around 6V, and begins stealing current from opto-battery 100, limiting the voltage to 6V. As the voltages on the capacitors and gate-source of the MOSFETs 102 and 104 increase the MOSFETs 102 and 104 turn on. As the MOSFETs 102 and 104 turn on, the gate-drain voltage of each of the MOSFETs 102 and 104 begins to change, causing some gate current to flow into the gate-drain capacitance. The gate-drain capacitance of the MOSFETs 102 and 104 also affects the charging and turn-on speed of the MOSFETs 102 and 104. The longer it takes to energize these collective gate-source capacitances to 6V, the longer it takes to turn on the isolated MOSFET switching circuit. Further, the voltage could be much higher for the gate-drain capacitances, also increasing the switching time.
Phototransistor 116 in FIG. 1 is optional and used when the isolated MOSFET switching circuit is part of a protection circuit. When the switching circuit should protect, Phototransistor 116 will begin to reduce the gate-source voltages of MOSFETs 102 and 104 and begin to open the switch circuit. MOSFETs 102 and 104 are in the linear region when the circuit is in the protection mode. Phototransistor 116 must handle the current of de-energizing the MOSFETs 102 and 104 capacitances 106 and 108 from 6V to the voltage necessary for protection.
To turn off the switching circuit of FIG. 1, switch 110 opens, cutting off current to the LED 112, and opto-battery 100. A resistor in parallel with the current source (not shown for simplicity) discharges gate capacitances 106 and 108, eventually lowering the gate-source voltage of MOSFETs 102 and 104 and turning off the switch circuit.
Newer MOSFETs, however, typically have a high gate capacitance, and the switching circuit shown in FIG. 1 is slow to switch on these newer MOSFETs due to the higher gate capacitance.
One alternative to correct for the slow switching of the newer MOSFETs is to use a gate isolation transformer. The gate isolation transformer provides the required gate current to quickly switch on the MOSFET switching circuit. However, the gate isolation transformer requires complicated drive circuitry for the transformer, as well as a large and costly transformer itself. Another alternative is to include a floating power supply in the design using another winding on a supply transformer. The winding provides the high current at high signal voltages. Although this type of design is less complicated than adding a gate isolation transformer, it is still a major redesign and added cost.
The disclosed technology addresses these limitations of the prior art.